Engines may be configured with various fuel systems for delivering a desired amount of fuel to a combustion chamber. Example fuel systems may include port fuel injectors for delivering fuel into an intake port upstream of a combustion chamber, and direct fuel injectors for delivering fuel directly into the combustion chamber. Still other engines may be configured with a dual fuel injection system that includes each of a port fuel injector and a direct fuel injector for each engine cylinder. The different fuel injection systems provide different advantages. For example, port fuel injectors may be operated to improve fuel vaporization and reduce engine emissions, as well as to reduce pumping losses at low loads. As another example, direct fuel injectors may be operated to improve engine performance and fuel consumption at higher loads. Dual fuel injection systems are able to leverage the advantages of both types of fuel delivery.
As such, there may be operating conditions where engines configured with dual fuel injection capabilities operate for an extended period with one of the injection systems inactive. For example, there may be conditions where the engine is operated with port injection only and the direct injectors are maintained inactive. The direct injectors may be coupled to a high-pressure fuel rail downstream of a high-pressure fuel pump. During the extended periods of non-operation of the direct injectors, the presence of a one-way check valve may result in high-pressure fuel being trapped in the high-pressure fuel rail. If the stagnating fuel is exposed to higher temperatures (such as higher ambient temperatures), the fuel may begin to expand and vaporize in the fuel rail, resulting in an increased fuel pressure, due to the closed and rigid nature of the fuel rail. This increased fuel temperature and pressure may in turn affect the durability of both the direct fuel injectors and related fuel hardware, in particular when the direct fuel injection system is enabled again. In addition, metering errors may occur when the direct fuel injector is re-enabled.
Example attempts to address direct fuel injector degradation due to stagnant fuel include activating a second injector in response to a fuel rail temperature increase. One example approach is shown by Rumpsa et al. in U.S. 2014/0290597. Therein, when operating an engine cylinder with fuel from a port fuel injector and not a direct injector, the direct injector is activated in response to a fuel temperature or pressure increase at a direct injection fuel rail. Fuel is then injected from the direct injector, while continuing to maintain engine combustion via port injection, until the fuel rail pressure and temperature is under control.
However, the inventors herein have recognized potential issues with such an approach. For example, as the pressure of fuel stagnating in the direct injection fuel rail increases, the minimum amount of fuel mass which is injected into the cylinder from the activated direct injector also increases. This can result in a larger than desired fuel mass being injected when the direct injection fuel system is re-enabled. As a result of the metering error, the engine may run at an air-fuel ratio that is richer than desired, increasing engine emissions, reducing engine stability, and degrading fuel economy. Additionally, there may be increased NVH issues. Still further, injecting a predetermined amount of fuel (e.g., injecting for a predetermined amount of time or directly injecting a predetermined fuel mass) may include injecting with a large proportion of direct injection to port fuel injection, thereby resulting in degraded engine performance.
In one example, the issues described above may be addressed by a method for an engine, comprising: while operating an engine cylinder with fuel from only a first injector, transiently opening a second injector to inject fuel into the cylinder; estimating a mass of the injected fuel mass based on a parameter of the injected fuel; and closing the second injector when the estimated mass is below a lower threshold, the lower threshold adjusted based on one or more engine operating conditions. In this way, fuel system hardware damage is averted.
As one example, during conditions when an engine is operated with port injection only, a direct injector may be intermittently activated and deactivated to maintain a minimum fuel injection mass of the direct injector within a desired range. Specifically, while maintaining a high-pressure fuel pump disabled, a minimum fuel injection mass from the direct injector may be estimated based on fuel parameters, specifically fuel temperature and pressure, of fuel in the direct injection fuel rail. As the temperature and/or pressure of fuel stagnating in the direct injection fuel rail increases, the minimum fuel injection mass may also increase. A cylinder direct injector may be selectively activated when the estimated minimum fuel injection mass reaches an upper threshold. Fuel may then be injected from the direct injectors until the minimum fuel injection mass reaches a lower threshold. Further, the lower threshold may be adjusted based on operating conditions while maintaining the lower threshold above a level where the high-pressure fuel pump needs to be re-enabled. For example, the lower threshold may be may be adjusted based on exhaust soot levels, engine knock or pre-ignition history, etc.
The technical effect of selectively opening and closing the direct fuel injectors based on a varying minimum fuel injection mass of the direct injector is that the direct injector may be able to inject small fuel masses when the direct injection system is re-enabled. In addition, hardware damage to the direct injection fuel system is reduced. By maintaining the minimum fuel injection mass within a desired range, fuel metering errors due to the injection of more fuel than commanded is reduced, specifically when smaller fuel injection amounts are commanded from the direct injector. In addition, the need to operate the high pressure fuel pump to deliver fuel via the direct injector is reduced. By prolonging a duration that the engine can operate with only port fuel injection and with the high pressure fuel pump disabled provides additional fuel economy benefits and reduces NVH issues.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.